Can my line manager give some of my duties including line management responsibility to somebody else?

I have had a number of issues with my line manager (which I have discussed in the hope of resolution but this has not come about). My line manager frequently approaches in a direct manner staff that report to me. My staff then respond to my line manager. I am constantly being by-ped and feel undermined and without authority over my staff and department. I am off sick for 2 weeks and my line manager contacted me to say that they would be getting extra help in to cover for the period. However I feel this is a front to effectively side-line me further. I believe that areas of my work will be handed over to this individual and my direct reports will begin to by-p me even more reporting into this new person and that my line manager will see them as the main contact between myself and my staff and not me. I would be grateful if anybody could share their views on this particularly in terms of legalities. My JD clearly states my role is to line manage my departments including the departmental managers. Thank you.